It is has been established for some time that isotopic analysis of compounds provides additional information extending beyond traditional structural and chemical analyses. While quantitative and qualitative structural analyses identify the chemical composition or structure of individual compounds, isotopic composition provides still further information concerning the origin, cycling and fate of these compounds.
In sports drug testing, compounds such as the steroid hormone testosterone are screened for, identified and quantified using conventional analytical techniques. The origin of the hormone is confirmed using 13C isotope measurements that discriminate between the endogenously produced compound, and an illegally administered synthetic equivalent.
There are further applications in ecology where the stable C, N, and S isotopic compositions of biota are used to determine the relative trophic positions of major organisms and examine the spatial and temporal changes in food web structures across ecosystems. This is especially important with respect to the effect on food webs of anthropogenically derived nutrients and contaminants from agricultural land uses.
Additional applications lie in oil exploration and oil spill identification where the isotopic compositions of large numbers of compounds from various oil samples and reservoirs are used to correlate compounds to their source. In exploration this can assist in understanding petroleum generation and migration which can be used to locate other reservoirs. In oil spill identification (and with other environmental pollutants) several potential sources may exist. Isotopic compositions of a select set of compounds can identify a single source or, in the case that an intermediate isotopic value is obtained, be used for source apportionment.
In addition to stable isotopes there are radioisotopes such as 14C (radiocarbon), which forms naturally in the atmosphere at very low abundances due collisions between nitrogen gas and comic rays. Radiocarbon exists in CO2 gas and is taken up by biological organisms (e.g. plants) or anything else that uses or adsorbs CO2. Materials enriched with radiocarbon are also produced artificially by man in laboratories or, in the case nuclear weapons testing, from radioactive residues. The half-life of 14C is approximately 5700 years which imparts a useful characteristic in that it can be used for age dating of carbon containing materials. At natural abundance levels this is widely applied in archeology for ages up to 60,000 years. Biomedical research uses 14C enriched compounds as a tracer label to study drug metabolism. The low natural abundance of radiocarbon makes it easier to identify labeled metabolites and study the fate of drugs in an organism however, labeled compounds and the analysis techniques employed are often costly. Methods and apparatus that improve the speed, sensitivity and precision of compound specific 14C analysis may minimize the amount labeled sample required and enhance applications in the biomedical field. This also applies to natural abundance levels of radiocarbon, stable isotopes and their respective applications.
Compound specific isotope analysis is typically achieved using a combination of gas chromatography (GC) to separate individual compounds, and mass spectrometry to determine isotopic composition of each compound. Stable isotopes such as 13C, 15N, 2H and 34S are routinely analyzed using an isotope-ratio mass spectrometer (IRMS) system. Radioactive isotopes such as 14C that occurs in very low abundances (around one part in a trillion) require a specialized and selective accelerator mass spectrometry (AMS) system. Compound separation and isotope analysis can be performed either “offline” where components are separated, collected and analyzed individually or “online” where the two systems are connected and analytes flow continuously from one to the other. The benefits of online analysis include: faster analysis times, reduced labor requirements, and increased sensitivity and precision resulting from the direct delivery of smaller amounts of analytes from the gas chromatograph to mass spectrometer.
However, matching a gas chromatograph to a mass spectrometer system for online isotope analysis presents a significant technical challenge. In particular, it may be necessary to combine two individual systems that function well separately with an interface. Both have optimum operating conditions that are not always compatible. Specifically, the carrier gas used to optimally transmit the analytes through each system may be different, and generally, may not match in flow rate. For example, the carrier gas used in a gas chromatograph may be hydrogen or helium, whereas the carrier gas for an AMS system may be Argon. Moreover, a gas chromatograph may supply compounds that are intact whereas, for high precision isotope measurements, AMS and IRMS systems may be configured to accept analytes as CO2 or N2 gas. Although, scientists have developed systems for interfacing a gas chromatograph with conventional mass spectrometers, these systems do not work well when used in conjunction with an AMS or IRMS system. For example, scientists use palladium tubing/membranes to separate out hydrogen, which is often the carrier gas for a chromatograph. However, palladium tubing poses several problems including absorbing some of the analyte and thereby diminishing sample output.
Accordingly, there is a general need for systems and methods for performing compound specific isotopic and chemical analysis. More specifically, there is a need for apparatus and methods for interfacing a gas chromatograph with mass spectrometers, and for preparing samples for AMS and IRMS systems.